This invention relates generally to particle-resonant-type oscillators, and more particularly to oscillators which produce radiation by direct conversion of the kinetic energy of an electron beam to radiative energy.
Many fundamental effects involving radiative emission from electron beams have been discovered and investigated, leading to the development of a whole class of free electron lasers and masers. Some of these effects are the Cerenkov radiation (emission from electrons passing in or near a dielectric material), bremsstrahlung (emission from decelerated electrons), Smith-Purcell radiation (emission from electrons passing near an optical grating) and cyclotron resonance radiation (emission from electrons in a uniform magnetic field).
Any electromagnetic oscillator must use a resonator structure to provide the feedback for oscillation. Electromagnetic wave oscillators (lasers or masers) based on the above effects are usually operated with a conventional Fabry-Perot resonator in the optical wavelength regime, or with a conventional cylindrical microwave cavity in the near millimeter and microwave regimes. These resonator structures have the following disadvantages:
(a) In order for the electron beam to enter the active region where the interaction between the electron beam and an electromagnetic wave takes place, the electron beam is usually magnetically deflected around the resonator mirrors (in the optical case) or a hole is opened in the front and end reflectors (in the microwave-near mm region). Both solutions degrade the operation of the device and introduce technological difficulties.
(b) When very short electron beam pulses are used, the effective Q factor of conventional Fabry-Perot resonators is low because the electromagnetic wave can traverse back and forth along the resonator only few times before the electron beam pulse passes entirely through the resonator and the gain stops.
(c) Conventional resonators usually may support many modes, particularly when the dimensions of the resonator are large relative to a wavelength. This causes a wide angular spread of the emitted radiation, lack of monochromaticity and coherence, and reduced efficiency.
(d) Specifically, for the cyclotron resonance maser (gyrotron), mode selection is made by use of a small cavity close to the wavelength dimension (.about.cm). This is possible because of the very high gain (growth rate) available and because the cross section of the beam can easily be made small relative to a wavelength. When higher frequency operation is desirable and particularly when one would like to operate at high harmonics of the cyclotron resonance frequency, it is difficult to use cavity dimensions which are small compared to a wavelength both because of the shorter wavelength of the radiation and the low growth rate encountered in harmonic operation. Furthermore, the large cavity can simultaneously support waves at the desirable high harmonic frequency together with lower harmonic frequencies. In this case the lower harmonic frequency (which usually is endowed with a higher growth rate) dominates the mode competition process in the laser and extracts most of the available electron beam energy. Thus it is not possible to get efficient operation at high harmonic frequencies. Operation at high harmonic frequencies is desirable not only to get short wavelengths (near mm. regime) but also to permit the use of permanent magnets instead of high-induction superconducting magnets. The latter suffer the disadvantages of being expensive, cumbersome, and require liquid helium cooling.